The present invention relates to phase locked loops and, more particularly, to systems and techniques for controlling the AC loop gain and DC prepositioning of such loops.
Phase locked loops and circuits for implementing such loops are well known in the prior art for providing frequency selective outputs which are easily controlled, adjustable, and stable in operation. In particular, a phase locked loop generally includes a voltage controlled oscillator having an output frequency which is dependent upon the voltage applied to a control input to the VCO. The output of the voltage controlled oscillator is coupled to provide a selected frequency as a desired output and is also provided as feedback to one input of a conventional phase/frequency discriminator. The phase/frequency discriminator has a second input which receives a reference frequency and provides an output which is coupled to provide a DC control signal to the control input of the voltage controlled oscillator.
In the above construction, the feedback of the output of the voltage controlled oscillator is compared in the phase/frequency discriminator with the input (reference) and that output signal used to control the VCO such that the output frequency of the VCO is locked to the reference frequency. In this manner, the phase locked loop can be constructed to provide an output frequency which tracks the input signal with a high degree of accuracy.
In accordance with prior art techniques, there have been proposed various methods for affecting the AC gain of the phase locked loop. There have also been proposed techniques for providing DC prepositioning in such phase locked loops. With the advent of digital systems, there has been a demand for improved frequency control using such phase locked loops and in particular, less complex, easily implemented and easily controlled digital phase locked loops. Such systems are particularly important to improve accuracy and allow easy and fast loop tuning and control.
The present invention has therefore been developed to overcome the specific shortcomings of the above known and similar techniques and to provide an improved tunable phase locked loop.